Worldwide known recoverable reserves of nonrenewable (fossil) energy sources amount to about 1200 Mrd. t BCU (BCU=bituminous coal units=29.3 109 J). The principal energy carrier is coal. It makes up 49.2%, of which 43.5% is hard coal and 5.7% is lignite. Thus processes for driving coal-fired gas power plants and steam power plants (G & S power plants) as well as other modern power plants have been developed to use pressurized coal dust firing technology (PCDF technology). The PCDF technology utilized in G & S power plants does not utilize natural gas, but rather coal under high pressures (10 to 20 bar) and burns the coal to produce hot gas under pressure which is used to drive a gas turbine. Because of the mineral content of the coal, the resulting flue gas must however be cleaned before it can be used in the gas turbine. Liquid slag particles and their main component, SiO2, especially must be removed. Up to now for this purpose a direction-change system has been provided in which the flue gas impinges on the surfaces of ceramic materials and the slag which there agglomerates can flow off in a liquid film which is removed by gravitation from the surfaces and thus from the flue gas stream. The deflection system includes precipitating devices for the slag, especially in the form of baffle plates or ball packings. For the liquid ash separation at the present time Cr2O3 or melt-cast Cr2O3-containing ceramics are used since Cr2O3 is the single ceramic which has sufficient corrosion-resistance in contact with a flowing coal slag. The stability of the chromium oxide derives from the fact that Cr2O3 undergoes no chemical bonding with the main component of the coal slag, SiO2, and does not form ternary mixed oxides. Similar characteristics with respect to reactions with SiO2 have been recognized previously only for UO2 and ThO2, but because of their radioactivity, these compounds have not been used.
A problem with Cr2O3-containing ceramics is that they, under power plant conditions and high steam pressures, tend to evaporate, whereby in an oxidizing atmosphere the volatile species Cr2O3 and CrO2(OH2) arise. Under PCDF conditions, the vaporization is yet more intensive since there a significantly higher temperature prevails on the one hand and on the other the chromium evaporation is about an order of magnitude more intensive because of the sodium oxide and potassium oxide of the carbon slag so that Cr2O3-containing ceramics also because of a cancer-producing effect of vaporized Cr6+ compounds cannot be treated as appropriate ceramics for liquid ash separation over longer periods.
Especially for the PCDF technology, ceramic materials are required that have the following characteristics:
1) Thermodynamic stability and corrosion-resistance at temperatures between 1250 and 1500° C. in an oxidizing atmosphere.
2. Low solubility and chemical stability in contact with a flowing coal slag with constituents like: SiO2, Al2O3, CaO, Fe2O3, MgO, K2O and Na2O.
3. No formation of volatile toxic products.